Why doesn’t the engine switch off during EcoRoll?

In order to switch off the engine while driving, the operational readiness of the safety-relevant vehicle systems must be ensured by an alternative energy supply. 

In addition, the engine must endure the increased number of starts without damage.

Have you ever wondered why the engine isn’t simply turned off when the truck is in EcoRoll mode?

The EcoRoll Function of the powertrain shifts the transmission to neutral when the truck’s inertia can be used to save fuel.

It would seem logical to shut the engine down at this time. Then, there would be no idle consumption while rolling and this increases fuel economy even more.

That’s a compelling argument for shutting off the engine, but then why isn’t it done?

If the vehicle does not meet these requirements, the engine must remain switched on.

Is it because it’s an impossible task?

No, it is not! It’s a question of will and cost.

In this chapter, you can read about everything that needs to be done before the engine can stop while the vehicle is still in motion.

This allows you to form your own opinion as to whether it makes sense.

EcoSail – a name for the EcoRoll engine switch-off function

I know the  EcoRoll function with engine shutdown  under the name  “EcoSail”

I think that’s a logical name. – The captain of the sailing ship also switches off the combustion engine when the ship is under sail.

What the wind is for a sailing ship is the inertia for a truck.

To differentiate between EcoRoll with and without engine shutdown, I’ll use the name EcoSail from now on.

So, dear OEM, since you’re the first to offer this feature, please consider calling it EcoSail!

If you choose a different name, I’ll have to rewrite the entire article 😉

Benefits of EcoSail

EcoSail improves the fuel-saving potential of EcoRoll in two respects.

No idle consumption when rolling

In my article on how EcoRoll works, it is mentioned several times that idling consumption during rolling wipes out a considerable part of the fuel savings.

This idle consumption is eliminated with EcoSail and the full diesel saving is realized.

The illustration shows the energy (respectively work) on the route on which the vehicle coasts from the upper speed to the lower speed with EcoRoll.

• The green color indicates the kinetic energy released, which propels the vehicle free of charge.
• Black stands for the energy that the engine absorbs during coasting and which must therefore be replenished afterwards.
• The red color symbolizes the waste heat generated in the engine when the missing energy is regenerated.
• The yellow bar is the idle consumption during the EcoRoll phase.

As you can see in the picture, EcoSail utilizes the entire kinetic energy for propulsion without consuming any additional diesel.

How much additional fuel will EcoSail save?

Let’s assume the idling consumption is between 2 and 3 liters per hour.

One minute less idling saves between 33 and 50 cm³ of diesel.

That’s equivalent to a double shot of liquor. (40 cm³).

If you want to know your real fuel savings, you need the current duration of the EcoRoll rolling in your application. However, this varies greatly, as it depends on widely fluctuating parameters.

Have a look at your watch and see how long your truck rolls in EcoRoll mode.

Using this time information, you can easily calculate how big the potential additional savings from EcoSail could be in your application.

Take the time in minutes and multiply it by the idling consumption per minute. From cm³ you get liters if you divide by 1000.

Simplified control saves additionally.

With EcoSail there are no bad rolling phases.

This makes the operating strategy for this driving function much simpler.

But, what are bad rolling phases?

In the case of EcoRoll, braking is particularly bad for fuel consumption, because extra idling consumption was previously invested to prevent the engine braking effect, and now the vehicle brakes after all.

The idling consumption was therefore unnecessary.

If there is active braking at the end of an EcoRoll phase, it is smarter not to disengage the gear, but to let the engine brake without diesel injection.

This reduces or avoids active braking and no idling consumption occurs.

In this case, engine coast is the more effective operating strategy.

If the EcoRoll phase is ended by braking, fuel consumption is higher with EcoRoll than without. This is stupid and must be prevented.

However, this means that you or your cruise control must already know at the start of the rolling phase whether it will end later with or without active braking.

As you can imagine, this is complicated to compute.

Good predictive cruise control systems can do this, but even with these, there are certainly some errors.

With EcoSail, it is irrelevant whether or not the brakes are actively applied at the end of the roll phase.

The control logic is therefore considerably simpler and mistakes have no negative effects.

Regardless of how the rolling phase ends, if the vehicle can roll, then it should do so in order to really catch all the potential savings.

Technical requirements to enable EcoSail.

Let’s now move on to answering the question posed above:

What requirements must a vehicle fulfill if the engine is to be switched off while driving?

I will discuss the important areas in which conditions must be met to enable the engine to be switched off while driving.

These areas are:

1. Durability and service life of vehicle components.
2. Operational readiness of the vehicle systems.
3. Conformity with applicable regulations.

1. An increase in engine wear must be avoided.

If the engine stops when rolling, it must of course be restarted at the end of the rolling phase.

This many additional engine starts can cause increased engine wear, which can result in premature failure of the engine or its components.

This must not happen under any circumstances.

It is generally known that engine starts are stressful for a combustion engine.

But which parts are at risk and how can this be countered?

I’ll start with a component that you might think of first.

Will the starter motor last?

Yes, the starter motor should not be a problem.

With EcoSail, the engine can be started at the end of the rolling phase using the clutch.

The vehicle has sufficient kinetic energy to accelerate the engine from a standstill to operating speed.

Therefore, the following start procedure can be used:

• engage the gear
• close the clutch gradually and accelerate the powertrain until its speed matches the wheel speed of the drive wheels.
• reactivate the injection and the journey continues smoothly.

Basically, it is “pushing” the engine. The buddy that pushes is the inertia force.

This means that the starter does not have to be used and therefore does not suffer increased wear.

The shaft bearings need to be taken care of!

The higher number of engine starts can lead to increased wear at the shaft bearings.

In a four-stroke engine, the large, highly stressed shafts (crankshaft, connecting rods, camshafts) are suspended in hydrodynamic plain bearings, and there is a very good reason for this.

These bearings offer very good efficiency, which is beneficial for fuel consumption.

In order to understand why these bearings react sensitively to starting processes, we need to know how they work.

At this point, I will explain the essentials very briefly.

How do engine shaft bearings work?

A shaft bearing allows the shaft to rotate relative to a stationary housing.

This relative movement tends to cause friction and friction in turn causes heat and thus energy loss.

It must therefore be the task of the engine designer to optimize the coefficient of friction at the bearing surface.

While the engine is running, this is achieved by the shaft floating on an oil wedge.

Fluid friction is the most efficient type of friction available to us.

However, hydrodynamic bearings only function in this manner when the engine is running.

The reason is simple. Since oil is a liquid, it flows out of the bearing and thus must be constantly replenished.

The engine’s oil pump is responsible for ensuring that there is always enough oil in the bearing. It continuously pumps new oil with pressure into the bearing gap.

The shaft picks up the oil on the non-loaded side of the shaft and draws it into the load-bearing area. This creates an oil wedge on which the shaft floats.

However, this highly effective and almost wear-free operating condition only exists when the engine is turning.

The oil pump needs the engine motion to supply oil and the shaft only forms the oil wedge when it is rotating.

What happens to the bearings, when the engine starts?

When the engine stops, the shafts and oil pump come to a standstill, causing the metal components of the shaft and bearing shell to make contact.

As long as the shaft is not moving, this is not a problem. Without movement, there is also no wear.

However, when the engine is started, the shaft now begins to rotate. The oil pressure must build up again and the oil wedge in the bearing must re-form.

This is where the bearing shells come into play.

The bearing shell is made of a special material that is optimized precisely for these conditions and can withstand this situation with little wear.

Mixed friction occurs for a few revolutions in this situation.

As the name suggests, mixed friction is a mixture of mechanical and hydraulic friction. The mechanical friction component causes wear on the bearing shells.

Bearing shells are wear parts that are designed for a pre-defined service life. If the service life is reached or exceeded, they must be replaced.

Replacing the shaft bearings is a costly operation and should therefore be carried out as late as possible.

The development engineer therefore designs the bearings so that they can withstand the number of starts that occur during operation under normal conditions.

However, EcoSail is significantly increasing the number of engine starts. The bearings must be capable of accommodating this increase.

Before the EcoSail function can be released for customer use, it must be ensured that the bearings can withstand the additional starting processes without premature failure.

2. Operational safety must be ensured.

As a driver, you must be able to control the vehicle at all times and mitigate any dangerous situation. Even if something breaks down.

This results in a very important requirement for the truck’s technology:

As long as the vehicle is in motion, all safety-relevant systems must function reliably at all times. The systems must not fail even in the event of a single malfunction. (Redundancy)

Fortunately, the days are gone when the driver had to operate the systems in a truck with their own muscle power.

However, as a consequence, the driver now relies on the engine to reliably provide the energy to drive the safety-critical systems.

The engine either feeds the energy directly into the system or charges an energy storage unit, from which the energy is then drawn as required.

We will look at how this is done for each system in detail in a moment.

Connecting the wheels to the engine provides redundancy during normal operation.

There was a good reason why the driving instructor told you: “Never shift into neutral while driving.”

Let’s take a look at why.

Suppose the engine breaks down.

The vehicle then continues to move until the kinetic energy is used up, i.e. until the vehicle comes to a standstill.

During this time, the steering and brakes must remain functional and the lights must not go out.

A safe operating state is then reached again at a standstill, where there is no risk of accidents anymore.

If the engine is still connected to the drive wheels, all vehicle systems continue to be supplied with energy by the engine, drawn from the kinetic energy of the vehicle.

When a gear is engaged and the clutch is closed, the engine is pushed by the vehicle’s weight until the kinetic energy is exhausted and the vehicle comes to a standstill.

Until then, the auxiliary consumers and therefore the systems will work.

Steering, brake, and electric systems are functioning correctly, allowing the vehicle to decelerate safely. The vehicle can be maneuvered to a safe location and parked there.

Once the vehicle has come to a standstill, the danger is over.

Certainly:

This safety strategy necessitates engaging the gears in the transmission, which is why the driving instructor made this serious statement.

How does EcoRoll deal with the redundancy?

Things get interesting with EcoRoll.

Here, a safety level is removed during the rolling phase because the motor is separated from the wheels.

However, the engine continues to idle, providing energy to the vehicle’s systems.

If the vehicle’s electronics detect a situation, where the idle does not supply enough energy, the gear will not be disengaged.

If anything should go wrong, a gear is immediately re-engaged in order to achieve maximum safety and keep the vehicle under control at all times!

Incidentally, this is also the reason why EcoSail is only used in conjunction with an automated gearbox.

The electronics, in combination with the automatic shifting devices, make sure to get the gear in quickly! You don’t want to be dependent on the driver in a situation like this.

With EcoSail additional measures are required.

With EcoSail, the drive train is opened and the engine stops. Without special measures, some systems would stop working.

Control of the vehicle could be lost. This must be avoided at all costs!

With EcoSail, additional measures must be taken to ensure operation while the engine is switched off and the drive train is open.

So let’s take a close look at this system by system.

The brake

The brake system of a truck is an external power system.

External power system means that the braking force is not provided by the driver. The driver only gives control signals.

In trucks, the brake actuating force is generated by compressed air.

The compressed air required is generated by the engine using a compressor and stored in several air tanks.

The brake is divided into several subsystems (brake circuits), each of which has its own reservoir.

If there is not enough compressed air in the air reservoirs, a strong steel spring at the wheel brake ensures that the brake is applied and the vehicle is stopped automatically.

The braking system operates independently of the engine as long as there is sufficient air pressure in the air reservoir. If not, the vehicle detects the situation and does not shut off the engine.

If the brake completely fails, the standard safety principle of the spring brakes activates as usual.

EcoSail is not a problem for the brakes.

The steering system

It’s a different story with the steering.

The steering is an auxiliary power system.

This means that the power comes from the driver and is only assisted by the vehicle.

The auxiliary power is generated through oil pressure in the steering gear. A power steering pump driven by the engine generates this oil pressure.

If the engine stops, the steering assistance is lost and the steering becomes unreasonably heavy.

We do not want to experience this situation. Anyone who has had the opportunity to perform the certification test for a steering system with failed power assistance knows what I’m talking about. (I have!)

So here, not only is the redundancy no longer present, but the steering simply does not function properly anymore.

Three alternatives are possible:

• Attach an additional power steering pump to the gearbox. (Dual-circuit steering)
• An electric power steering pump.
• A steering assistance system with an electric motor on the steering gear.

The first solution has been common for heavy vehicles for many years. If the axle load is so high that the legal requirements for mechanical redundancy are not met, a dual-circuit steering system is usually installed.

If a power steering pump is connected to the gearbox output, it is not disconnected from the wheels when neutral is engaged. It is then driven by the kinetic energy of the vehicle until the wheels come to a standstill.

In addition to ensuring steering assistance when the engine is not running, the second variant also offers the option of controlling the oil volume flow as required, thereby saving even more fuel while driving.

I would recommend the third alternative. With such “active steering”, in addition to the steering assistance during EcoSail, it also offers the possibility for active lane keeping or semi-automated driving. I see many advantages combined here.

A whole range of vehicles are already equipped with such steering systems, which would enable this redundancy level.

With EcoSail, the steering system must be equipped with additional steering assistance.

The electrical system

If the electricity were to fail during the journey, the consequences would be serious:

• The lights go out and the driver can no longer see what is happening in front of the vehicle at night.
• The signaling system fails, no tail light, no turn signal, no hazard warning lights.
• The assistance systems no longer work, even though they might be urgently needed in precisely this situation.
• Electric actuators or servomotors no longer work.
• It is also no longer possible to engage the gear.

The vehicle electrics and electronics must therefore function in any case.

Each truck has two independent power sources. The generator and the batteries.

The generator is driven by the engine and generates electricity as long as the engine is running. It supplies the vehicle’s electrical system and also charges the batteries.

The starter batteries supply the vehicle with electrical energy also when the engine is not running. The best-known case is undoubtedly the engine starting process.

The power for the starter motor comes from the battery. However, they also buffer consumption peaks while driving.

In the event of a generator fault or if the drive belt on the generator breaks, the batteries supply the vehicle and guarantee safe operation.

Here, too, we have a situation where the generator does not supply any power during an EcoSail rolling phase.

With the starter battery, only one source of electrical energy remains on the on-board power supply.

Before EcoSail is activated, it must therefore be ensured that the battery charge level is sufficiently high.

The vehicle is still working properly, by there would be no redundancy anymore.

Unfortunately, it is not uncommon for starter batteries to fail unexpectedly.

If this occurs when EcoSail has disengaged and stopped the engine, it would be fatal.

A vehicle with EcoSail needs an additional battery to take over the power supply if the starter battery fails.

It is conceivable that the battery is large enough to supply the vehicle with power until it comes to a standstill.

Another alternative would be a battery that is sufficient to put the vehicle back into gear, restart the engine, and reconnect the alternator to the onboard power supply. The alternator can then take over again.

The backup batteries must be equipped with battery monitoring to prevent potential breakdowns that may go unnoticed due to their infrequent use.

If a failure occurs and goes undetected, the backup batteries would also be rendered inoperable, thus failing to fulfill their intended function. To prevent such occurrences, it is essential to monitor these batteries closely and promptly display any signs of failure.

3. The engine emission values must be complied with.

Exhaust gas after treatment is another engine system that must be checked for EcoSail operation and adapted if necessary.

For the exhaust gas after-treatment chemistry to work perfectly, the catalytic converters in the exhaust box must have a certain operating temperature. If the temperature is not sufficient, the emissions will be increased.

The exhaust gas legislation has defined very precise requirements for this and these must be complied with in any case.

During engine operation, the hot exhaust gas provides the necessary catalytic converter temperature.

If we switch off the engine with EcoSail, this hot exhaust gas flow is missing. At low outside temperatures, this could result in the catalytic converter temperature falling below the required level. The result would be excessively high exhaust gas values.

Before the temperature in the exhaust gas catalytic converter falls below the critical value, the engine must be restarted to keep the exhaust gas values within the required range.

Let’s hope that the catalytic converters are well-insulated to prevent this from happening.

Summary

I hope it won’t be long before we see a vehicle with EcoSail on our roads!

If you have not yet read the articles on EcoRoll, its influencing factors, and Pulse&Glide, you should do so so that you are familiar with the entire subject area.